Product Key Testing

In addition to activation, Windows XP service packs will refuse to install on Windows XP systems with product keys known to be widely used in unauthorized installations. These product keys are intended to be unique to each boxed (or bundled) copy of Windows XP and are included with the product documentation, but a number of product keys were posted on the Internet and were then used for a large number of unauthorized installations. The service packs contain a list of these keys and will not update copies of Windows XP that use them.
Microsoft developed a new key verification engine for Windows XP Service Pack 2 that could detect illicit keys, even those that had never been used before. After an outcry from security consultants who feared that denying security updates to illegal installations of Windows XP would have wide-ranging consequences even for legal owners, Microsoft elected to disable the new key verification engine. Service Pack 2 only checks for the same small list of commonly used keys as Service Pack 1. This means that while Service Pack 2 will not install on copies of Windows XP which use the older set of copied keys, those who use keys which have been posted more recently may be able to update their systems